The present invention relates to a process for the reversible sorption of sulfur trioxide on a recyclable sorbent, and to the composition of sorbent and sulfur trioxide.
Today the major uses of sulfur trioxide are in sulfonations and the manufacture of explosives. A more recent and relatively smaller volume use is in the electronics industry where very pure sulfur trioxide can be used for etching of electronics parts. Levenson and Waleh describe this application in U.S. Pat. No. 5,763,016, where exposure of the electronics parts at room temperature to 400xc2x0 C. to dry gaseous sulfur trioxide is used to etch organic coatings, films, and layers, including photoresists. In this application, the delivery of small quantities of pure sulfur trioxide is necessary. The electronics industry is typically not equipped to handle sulfur trioxide in bulk liquid form, or the consequences of safety incidents with bulk quantities of such a toxic and corrosive in gas or liquid form.
Liquid sulfur trioxide should be stored at a fairly precise temperature range of 35-41xc2x0 C. to maintain a liquid state and keep it from freezing. Also, to avoid the formation of alpha and beta forms, it is necessary to store the sulfur trioxide in the presence of a polymerization inhibitor. Temperatures of 35-41xc2x0 C. result in a significant sulfur trioxide vapor pressure and thus require storage in a pressure vessel. If sulfur trioxide were sorbed on a substrate, it could be stored under an inert atmosphere at room temperature in a non-pressure container made of a suitable material such as stainless steel or glass. No inhibitor would be required to prevent polymerization.
Tsvetkov, et al., in Khim. Ind. (Sofia) (1987), 59(8), 356-9, Chem. Tech. (Leipzig) (1995), 47(5), 252-4, and Deposited Doc (1983), Issue VINITI, pp. 2873-2880 (1980) describe the sorption of sulfur oxides on acid-modified natural bentonite and on the natural zeolites mordenite and clinoptilolite (having a SiO2/Al2O3 ratio corresponding to a Si/Al ratio equal to 4.5-5.025). They noted the catalytic effect on conversion of sulfur dioxide to sulfur trioxide and that desorption of sulfur trioxide formed in the sorbent was reversible and not accompanied by chemical transformations. Sorption capacity fell by 20-30% in the first 7-8 cycles in the bentonite example. Kel""tsev et al. (Russian J. Phys. Chem. 44(6), 1592-1594, 1970) describe the sorption of sulfur trioxide by hydroxylated and dehydroxylated silica gel.
Typically, the cyclic sorption of sulfur trioxide by acidic zeolites such as mordenite deal with zeolites that require relatively high temperatures of greater than 400xc2x0 C. for complete desorption. They show a pronounced decrease in activity after several cycles.
It is desirable to have sulfur trioxide in a safer and more easily used form, such as reversibly sorbed on a substrate that allows easy desorption at lower temperatures and thus simple delivery of the sulfur trioxide. Also, it is desirable to have the sorbed sulfur trioxide in an easily flowable form, for instance as free-flowing pellets, available in a commercial quality (typically 98% minimum) for conventional uses or in a highly purified quality for use in the electronics industry (typically 99.9%). The desorbed sulfur trioxide should be no lower in purity than the sulfur trioxide feedstock. Furthermore, it is desirable for the sorbent to be reusable. The present invention provides such an sorbent and a process for its use.
The present invention comprises a process for reversible sorption of sulfur trioxide onto a sorbent comprising a) contacting from about 15% to 100% sulfur trioxide with the sorbent under anhydrous conditions at a temperature of from about 35xc2x0 C. to about 150xc2x0 C. thereby sorbing the sulfur trioxide onto the sorbent, b) desorbing sulfur trioxide from the sorbent at a temperature of from about 150xc2x0 C. to about 350xc2x0 C. at about atmospheric pressure, or under a vacuum pressure, and c) recycling said sorbent by continuously repeating steps a) and b), wherein said sorbent has a pore size of at least 0.5 nm and consists essentially of silica or zeolite, said zeolite having a silicon to aluminum ratio in the ranges of from about 1 to about 4.4 or greater than about 5.1.
The present invention further comprises a sorbent consisting essentially of silica or zeolite, said zeolite having a silicon to aluminum ratio in the ranges of from about 1 to about 4.4 or greater than about 5.1, said sorbent having a pore size of at least 0.5 nm, and having adsorbed thereon a minimum of about 1% by weight sulfur trioxide.